Many telecommunications devices include backplanes for transmitting digital information between components of the devices. For example, a telecommunications switching system might include a backplane for transmitting digital data representing voice signals between cards associated with incoming and outgoing ports. Typically, such a system would also include a mechanism to allow the system to detect a timing error, loss of clock synchronization, or other clock failure associated with the total or partial failure of one of the cards or of its clock generation functionality. Successful operation of the system in many instances will depend heavily upon the ability of this mechanism to detect and respond appropriately to such a failure to meet often stringent availability, flexibility, and other requirements placed on the system.
As the telecommunications industry continues to dominate the growth of the global economy, meeting availability, flexibility, and other requirements placed on switching and other systems has become increasingly important. High availability is generally considered as exceeding 99.999 percent availability, amounting to less than approximately five minutes of “down time” each year, and generally requires a system to be able to detect and to autonomously handle certain faults, such as a clock failure associated with a card or its clock generation functionality, without immediate human intervention. Providing high availability is often a de facto if not explicit competitive requirement for many telecommunications manufacturers.
However, prior techniques for detecting and responding to clock failures are often inadequate to meet high availability and other requirements. One such technique involves monitoring a reference clock signal and, in response to a loss of the reference clock signal, initiating a delayed or even a “hard” switchover to a redundant reference clock signal. Hard switchovers of this type often require significant time to accomplish and may result in “slips” in the network, lost calls, and other losses of data integrity. Even a delayed switchover may be of little use if the secondary reference clock signal has also been lost or is otherwise unsuitable. Prior techniques often do not allow the system to continue operating, uninterrupted and maintaining substantial data integrity, despite such clock failures. Moreover, although a system using such a technique might raise an alarm to indicate the clock failure, before or after initiating the switchover, the system might not be able to determine the source of the error—either the source of the reference clock signal or the card itself—which may lead to unnecessary switchovers and other undesirable consequences. These and other deficiencies are particularly apparent in high availability backplane environments of telecommunications devices.